Effect of Temperature on Engine Performance
It is well known that the efficiency of the internal combustion engine is greatly affected by temperature. It is for this reason that a major modification of the engine cooling system may have a first-order effect on engine performance. In general internal combustion engines, whether diesel or spark-ignition, are "heat engines" and operate more efficiently when hot. Accordingly, current design convention seeks to provide for attainment of temperatures of the walls of the cylinder bores at as high a level as possible. For this reason present-day liquid-coolant systems are operated under pressure. Pressure raises the boiling point of the liquid, and accordingly the coolant may be operated at higher temperatures without "boiling over."
In conventional cooling systems, however, there is a penalty for high bore temperatures--temperatures at the cylinder head are also increased. This tends to cause premature ignition of the fuel charge, which most drivers recognize as "knocking", and localized heat damage such as metal cracks. Further insight into temperature effect is gained from consideration of what happens to the energy of the fuel supplied to the engine of an automobile. It is roughly as follows:
Heat rejection in the exhaust gas--33% PA0 Heat rejection in engine cooling--29% PA0 Indicated horsepower--38%
The indicated horsepower is partly consumed by pumping gases into, through and out of the combustion chambers and out the exhaust pipe (6% of total energy input), piston ring friction (3%), and other engine friction (4%), leaving an engine brake horsepower of 25% of energy in. In the case of automobiles, by far the largest field of use of internal combustion engines, only about one-half of the brake horsepower is ultimately used to move the automobile. The other half is lost in coasting, idling and braking, in drive train friction and other losses and in powering accessories. About one-half of the energy at the wheels is used to overcome aerodynamic drag and the rest tire friction and hysteresis.
Engine temperature affects cylinder cooling heat rejection and thermodynamic cycle efficiency in various ways. Engine temperature also affects friction losses. The requirement in conventional vehicles of a radiator cooled by ambient air flow increases aerodynamic drag, relative to the more efficient body shapes that could be used if the cooling air intake for the radiator were eliminated.
Basic Engine Cooling Requirements
The primary purpose of an engine cooling system is to keep the engine within maximum and minimum temperature limits under varying loads and ambient conditions.
The combustion process in an engine causes excessively high temperatures around the mixture ignition areas, normally in the top part of the combustion chamber in piston engines, and exhaust valve seat and port surfaces. Excessive temperatures in these areas cause surface ignition, leading to engine knock, mechanical failures of engine materials, and increases in HC (hydrocarbon) and NO.sub.x (oxides of nitrogen) emissions. Excessive cooling of the engine adversely affects fuel consumption, exhaust emissions of HC and CO, deposits, and vehicle driveability. Temperature differences throughout the engine cause thermal distortion and stress, which lead to engine wear, leakage, and failure. The ideal cooling system, therefore, balances these factors in order to maintain a temperature that is high enough to promote fuel economy, minimize emissions, maintain driveability, etc., low enough to eliminate preignition and mechanical failure and uniform enough to eliminate thermal distortion and its resulting problems.
In addition to the cooling requirements for an engine operating under steady state conditions, as described above, a cooling system has further complicating requirements. The temperature of the engine has a tendency to increase with an increase in engine load. These load increases may be due to increased speed, road grade changes, additional weight in the vehicle, or many other causes. In addition, the ambient temperature increases have an adverse effect on engine temperatures since the temperature differential between the engine and the cooling air is reduced. For all of the above reasons, a cooling system which can maintain a uniform temperature in spite of varying engine loads and ambient conditions is the design objective.
Types of Cooling Systems
The radiative and convective heat transfer from combustion gases to the combustion chamber walls, the conductive heat transfer through the combustion chamber walls to other parts of the engine and the heat transfer area between the engine metal and the cooling system are all variables determined by engine design. As such, these factors are beyond the control of the cooling system design, and are assumed to be constant for purposes of comparison among various types of cooling systems.
Air Cooling Systems
Due to the low order of the heat transfer coefficient of air, a large volume of air flowing over the heat transfer area is required to reduce the temperature in an engine. This method of cooling is generally unsatisfactory in an automotive engine due to the wide variations in ambient conditions, e.g., ambient temperature and vehicle speed, and engine speeds and the difficulty in maintaining any control over engine temperature. As the vehicle speed increases, the volume of air flowing over the engine also increases, and as the vehicle speed decreases or the vehicle stops, the volume of air, even enhanced by a large fan, decreases; consequently, the cooling effect decreases. Additionally, finned areas create local hot spots between fin-to-bore contact points. It is difficult to maintain the engine temperature within required limits, thus making this cooling method ineffective for surface vehicles. Because air temperatures at high altitudes are very low, air cooling is generally satisfactory for aircraft, though there are advantages to be derived from liquid cooling of aircraft engines.
Liquid Cooling Systems
The liquid cooling system is the system most commonly used to control the temperatures in internal combustion engines. Conventional liquid cooling systems are pressurized, with forced circulation of a liquid coolant by means of an engine-driven pump. The closed loop system circulates the liquid coolant between the engine water jacket, where heat is transferred to the coolant from the combustion chambers, and a radiator, where heat absorbed by the coolant in the engine is transferred to air flowing through the radiator. A pressure relief valve in the radiator fill cap is set at a pressure high enough to raise the coolant boiling point, thus preventing the liquid coolant from escaping under the normal range of engine operating temperatures.
To reduce engine warm-up time, a thermostatic valve is located at the outlet of the engine water jacket. The thermostatic valve opens only when the temperature exceeds a predetermined value. At coolant temperatures below the preset value of the thermostatic valve, little or no coolant can flow to or from the engine, so that the temperature of the relatively small portion of the total coolant that is trapped in the engine jacket will rise rapidly, and the engine can operate more efficiently sooner after a cold start.
Although conventional pressurized single-phase liquid coolant systems are reliable and require relatively little maintenance, they have several inherent drawbacks. Surface convective heat transfer coefficients for a fluid in the liquid phase are relatively low and vary with flow velocity. In the typical automotive cooling system, cooled liquid from the radiator enters the engine water jacket at the lower front part of the engine, and heated liquid leaves from the top of the engine. Therefore, the front cylinders will run cooler than the rear cylinders. Also, it is difficult to maintain uniform flow velocity of the liquid coolant through the complex flow passages inside the cooling jacket, so local hot spots develop throughout the engine. These hot spots are believed to contribute to the production of oxides of nitrogen in the engine exhaust gases.
Since the highest temperatures are generated in the combustion chambers at the tops of the cylinders, and since the coolant flow is generally upward through the engine, the upper part of each cylinder is much hotter than the lower part. This temperature differential from top to bottom of the cylinder causes thermal distortion of the engine block and cylinder head with consequent increased blow-by and oil consumption. Another problem caused by top-to-bottom temperature differentials is that of wall quenching, which produces an unburned layer of gases on the relatively cooler lower cylinder walls. This is a source of excessive carbon monoxide and unburned hydrocarbons in the exhaust gases. It also results in poorer fuel efficiency. Additionally, liquid systems are highly sensitive to ambiant temperature changes on a directly proportionate scale.
Evaporative Cooling Systems
Evaporative cooling (known also as boiling liquid or ebullient cooling) of internal combustion engines has been known for at least seventy years and has been the subject of numerous efforts over those years to develop a system that fulfills the many functional requirements for engine cooling systems in a reliable, effective, low-cost, practical way. Despite those efforts boiling liquid cooling has had virtually no commercial application. Some automobiles with boiling liquid cooling systems were built in the 1920's, and boiling liquid cooling has been applied to some extent to stationary engines, such as those used in the drilling industry, within the last twenty-five years. Nonetheless, there are some generally recognized advantages to boiling liquid cooling.
One of the advantages of a boiling liquid cooling system is that the convective heat transfer coefficients for vaporizing and condensing the coolant are an order of magnitude greater than the coefficient for raising the temperature of a circulating liquid coolant without boiling. Therefore, the temperature of the coolant in an evaporative system tends to be virtually the same in all parts of the engine.
In typical boiling coolant systems, liquid coolant is boiled within the cooling jacket of the engine, and the vaporized coolant is withdrawn from the upper part of the cooling jacket and conducted to an air-cooled radiator or condenser, either directly or through a vapor-liquid separator tank. The condensate collects in a sump connected to the bottom of the condenser and is returned to the inlet of the engine cooling jacket or to a supply tank for gravity flow to the engine.
Since boiling occurs at a constant temperature (assuming constant pressure), and since surface convective heat transfer coefficients for fluids being converted to the vapor state are much higher than those for the same fluids kept in the liquid state, boiling liquid cooling systems can maintain cylinder wall temperatures more nearly constant from top to bottom. In addition, the entire cylinder wall will usually be hotter, thereby reducing the production of carbon monoxide and unburned hydrocarbons in the exhaust gases, reducing friction, and improving fuel economy.
There are, however, several disadvantages to conventional pressurized evaporative cooling systems. An inherent major problem is loss of coolant supply in those systems due to vapor loss through vents or pressure relief valves and greater risk of high pressure leaks in the system. Many vapor cooling system produce an excessive volume of vapor in order to maintain the engine at the desired temperature level (100.degree.-116.degree. C., 212.degree.-240.degree. F.). In a high pressure system, the condenser, where the vapor is condensed back to a liquid state, may restrict fluid flow, thereby causing back pressure and vapor build-up in the engine cooling jacket. This back pressure displaces the liquid coolant in the engine cooling jacket with vapor, and contributes to engine failure through loss of cooling in the region where vapor has displaced liquid phase coolant. A further problem with most previous systems is the need for condenser fans and circulating pumps, either mechanical or electrical. It is because of these and other problems that previous vapor cooling systems have not, since the early days of the automobile, been commercially used in automotive engine cooling systems and little used in other fields.
Particular Prior Art References
There is, of course, a substantial body of patent, technical and lay literature on the subject of boiling liquid cooling for internal combustion engines. A few of these documents warrant a brief discussion here, because certain of the embodiments of the present invention may utilize some of the concepts found in them.
One such concept is the use of a condenser, the condensing surface of which is constituted by an external skin panel of a vehicle. This idea is proposed for use in automobiles in Barlow U.S. Pat. No. 1,806,382, May 19, 1931 and for use in aircraft in Lynn et al., U.S. Pat. No. 1,860,258. The Barlow patent also describes the advantage of such a condenser of eliminating the need for a fan to blow cooling air through a tube condenser and of being able to provide a hood over the engine compartment that will reduce intrusion of dust and lessen release of fumes back toward the passenger compartment.
Another feature that is useful in the present invention is that the condenser be located at a level above the engine coolant jacket and that condensed coolant be returned to the jacket by gravity. This eliminates the need for a pump. Elevated condensers with gravity return of condensate to the engine are proposed in the Barlow patent and in Bullard U.S. Pat. No. 3,082,753.
The Basic Defect of Prior Art Systems
It is believed that a basic and fatal defect has existed in all previously proposed boiling liquid systems, namely that a major fraction of the coolant in the coolant jacket of the engine head is in the vapor phase during most operating conditions of the engine other than during warm-up. Universally, the coolant in the jacket of the engine head receives the vapor evolved from the coolant in the block. When vapor from the block is combined with the large amount of vapor evolved in the head, especially around the exhaust ports and near the dome of the combustion chamber, the total vapor content of the head coolant jacket is so high that there is insufficient liquid coolant available in places where it is most needed to extract heat by vaporizing, and hot spots develop and persist in the combustion chamber dome. The vapor in the head has little capacity to accept more heat, and vapor pockets tend to form near the hottest regions where they are the most damaging to effective heat transfer.
The problem of the presence of excessive coolant vapor in the head coolant jacket can be especially harmful in narrow portions of the jacket, such as above the exhaust ports and at the openings where the block jacket communicates with the head jacket. Even small projections on the walls of the jacket in these narrow passages can deflect the flow of liquid coolant and provide a site for a vapor pocket where a hot spot can develop and persist. These vapor pockets tend themselves to block or divert the flow of liquid coolant. Hence, the engine runs much of the time with a substantial fraction of vapor in the head coolant jacket and with insufficient coolant in the liquid phase for adequate heat transfer.
The fact that most boiling liquid cooling systems proposed and used in the past have produced a violently boiling effluent from the head, such that a lot of liquid coolant is expelled with the vapor and a vapor-liquid separation is needed, strongly suggest the presence of excessive vapor. More importantly, preignition (knocking), which is undoubtedly due to hot spots, has been a chronic problem in vapor-cooled engines--pre-ignition reduces efficiency and can cause severe engine damage and ultimate failure. This ultimately requires a retarding of the ignition spark lead (advance) for correction, which results in a loss of fuel economy. The hot spots also cause high thermal stresses that lead to cracking of the head.